In a chair of this kind, a mechanism is provided which assures that when the user shifts his weight, for instance when leaning back in the chair, an anatomically adapted shift in the relative inclination of the seat surface and the backrest is effected in a coordinated manner. Such mechanisms are therefore known as synchronizing mechanisms.
A chair of this general type, provided with this kind of synchronizing mechanism, is known from German patent document A 27 57 349. However, like other comparable chairs known previously (see, for instance, German patent documents C 27 33 322 and C 28 36 216 which correspond to U.S. Pat. No. 4,200,332), this chair has the disadvantage that because of the relative movement in shifting the inclination of the seat and the backrest, a pull is exerted on the clothes of the user, which is perceived as unpleasant. This is due, among other factors, to the fact that in the previously known construction, an adjusting device is supported on the supporting column of the chair base such that it is pivotable about a horizontal axis, with the pivoting movement of the seat and backrest being coordinated via a triangular swinging arm. The posterior end of the seat is connected via a toggle joint to the lower end of the backrest support column, and this articulation point of the toggle joint is connected to the lower articulation point of the triangular swinging arm via a gas spring. This means that the backrest support column, in the final analysis, is pivotably joined to a section of the seat via a parallelogram linkage embodied by the triangular swinging arm, the toggle joint, the gas spring and this section of the seat. A further parallelogram linkage is embodied between the triangular swinging arm and a toggle joint which with one end engages the vicinity of the front edge of the seat and the other end of which is joined to a further gas spring.
The known embodiment has not only the inherent disadvantage of a pushing or pulling effect on the user's clothes but also has the disadvantage that in order to attain a damping effect on the inclination shifting movement on the one hand and to attain a sufficiently great restoring moment on the other, two springs that are independent of one another must be provided, which makes construction relatively complicated and increases the cost of manufacture.